Add "create" prompt to open registration page

[moshenskyi]

Checklist

• I read the Contribution Guidelines
• I signed the CLA and WG Agreements
• I ran, updated and added unit tests as necessary.
• I verified the contribution matches existing coding style.
• I updated the documentation if necessary.

Motivation and Context

For a long time there were lots of hacks to open registration page directly. Now, we have a standard that is supported by a majority of authorization servers. That's why I'm extending AppAuth with another prompt value from OIDC spec (link) described in issue #1134.

Many users already use this prompt parameter, so the idea is to extend the library to:

• standardize the set of allowed prompt values
• add extensive documentation for newcomers with link to specs (as we do it for other values)
• align AppAuth with real-world OIDC implementations (Keycloak, Auth0, etc.)

Description

This solution allows to use predefined prompt to open registration page directly in AuthorizationRequest: